RF power amplifiers are important components widely adopted in radio access system and microwave system for amplifying signals. In a CDMA or WCDMA radio access multi-carrier system, when a power amplifier in a sector fails, cell phone users in the sector will be cut off, which imposes negative influence on the quality of service of mobile communication operators. Therefore, CDMA radio access multi-carrier systems have strict requirements concerning the stability and reliability of power amplifiers. However, power amplifiers work with strong current, high voltage and tough thermal environment, which make them vulnerable to failures. Hence it is necessary to develop a hot standby technology for power amplifiers in order to improve their stability.
Taking a 3-sector application as an example, as shown in FIG. 1, a sector of conventional technology has at least one power amplifier, which only amplifies input signals of its home sector and does not deal with signals of other sectors. Power amplifiers of different sectors are isolated from each other. The input signals from IN1, after being amplified by a power amplifier PA1, reach antennas S1 of a sector. The input signals from IN2 are amplified by a power amplifier PA2 and reach antennas S2. Similarly, the input signals from IN3 are amplified by a power amplifier PA3 and reach antennas S3. If PA1 is damaged, there is no signal output from its home sector and all mobile user terminals in the sector are unable to access the system, which will hurt the reputation of both the mobile operator and the equipment manufacturer. Such conventional solution without hot standby and power share system requires high reliability of power amplifiers.
There is a hot standby and power share network for power amplifiers in the prior art, as shown in FIG. 2. The solution (3 dB hybrid couplers) adopts hybrid matrix (taking 4×4 matrixes as an example), which consists of front hybrid matrix, power amplifier matrix, and rear hybrid matrix. The difference between such a solution and the conventional solution, which does not adopt hot standby and power share technology, is that each of the power amplifiers in the power amplifier matrix amplifies the input signals of all three sectors. The front hybrid matrix works as a power divider and the rear hybrid matrix is a power combiner. The work process of the solution is explained as follows. The input signal IN1 is first divided by the front hybrid matrix into four signals, which reach the input ends of the four power amplifiers respectively. The four signals have the same amplitude and keep a 90 degree phase difference to the signals of neighboring ports. After being amplified, the four signals are inputted into the rear hybrid matrix, which is structurally the same as the front hybrid matrix and are combined into one signal to be sent to Antenna S1 of Sector 1. Because there is a 90 degree phase difference to the signals of neighboring ports, the signal from IN1, after being divided, amplified, and combined, will be exported at Antenna S1 and not exported at Antennas S2 and S3 or applied to matching resistance in an ideal application. Similarly, the input signals from IN2 will only be exported at Antenna S2, and those from IN3 will only be exported at Antenna S3 without being sent to other ports.
Compared with the conventional solution which does not adopt hot standby and power share technology, the solution described in the fore-going paragraph can, theoretically and without regard to the signal losses in the dividing and combining process, increase the power of the signals exported to the antennas to 1.23 dB in every sector.
In such a solution, when a power amplifier fails, the remaining two power amplifiers still work normally and the input signals from IN1 can still reach Antennas S1 of Sector 1 after being amplified by the system, and the input signals from IN2 and IN3 also reach S2 and S3. The cell phone users in all three sectors can access the system, the failure of the conventional solution in which the cell phone users in a sector cannot access the system will not happen to this solution, and the hot standby and power share for power amplifiers is thus achieved.
However, in this solution it is required to divide a signal into four signals with exactly the same amplitude and a 90 degree phase difference in the front hybrid matrix, so that the signals from other sectors can be cancelled based only on the amplitude difference and phase difference between signals, which imposes high requirement on the signal amplitude and phase. The factors described below may cause incomplete cancellation of signals, which leads to cross-talk between sectors and deteriorates the sector isolation. This is a fatal defect, which prevents its widespread use. The following are the factors that affect the amplitude and phase difference between signals in the dividing and combining process and eventually cause incomplete cancellation of signals:
1. The coupling of the 3 dB hybrid couplers being unable to achieve the ideal of 3 dB due to flaws in their design and manufacturing process.
2. The 3 dB hybrid couplers, given any deviation from the center frequency, being theoretically unable to meet the requirements on signal amplitude and on signal phase difference at the same time, even when they are ideal 3 dB hybrid couplers which meet the requirements on phase and coupling degree only at the center frequency.
3. Additional phase difference produced by inconsistent electrical lengths of the transmission lines A1, A2, A3 and A4 in the front hybrid matrix.
4. Additional phase difference produced by inconsistent electrical lengths of the transmission lines B1, B2, B3 and B4 between the front hybrid matrix and the power amplifiers.
5. Additional phase difference produced by inconsistent electrical lengths of the transmission lines C1, C2, C3 and C4 between the rear hybrid matrix and the power amplifiers.
6. Additional phase difference produced by inconsistent electrical lengths of the transmission lines D1, D2, D3 and D4 in the rear hybrid matrix.
7. Inconsistent characteristics of power amplifiers PA1, PA2, PA3 and PA4; characteristics especially the gain and phase difference among the power amplifiers are expected to meet extremely strict requirements: the gain difference should be less than 0.5 dB and the phase difference should be less than 5 degrees, which means the power amplifiers must be selected and matched according to extremely strict standards.
It can thus be seen that the solution imposes extremely high, nearly excessive, requirements on manufacturing process and the consistency of devices. Otherwise it would be very hard for the isolation between sectors to reach 25 dB, a parameter which has already failed to satisfy the requirements of CDMA system protocols, not to mention the requirements concerning the adjacent channel interference in special sector configuration. Furthermore, when the system is running, if a power amplifier of the system is needed to be replaced, a replacement of exactly the same gain and phase must be found. This is usually another power amplifier from the same manufacturer and in the same batch with the original one. Otherwise the four amplifiers should be replaced all together with another selected matched four power amplifiers. The poor interchangeability of modules further limits the batch application.
Single pole double throw (SPDT) microwave switches are widely applied to RF systems or microwave systems for switching signals. Power dividers are also widely adopted in RF systems for dividing input power to multiple shunted circuits according to a certain power ratio, or for a reverse process, i.e., combining power. In a low frequency circuit, because the wavelength is much longer than that of transmission lines or components, the circuit is a lumped circuit in which equal electrical potential are maintained in the transmission lines all the time. However, the characteristics of transmission lines in the field of microwave transmission are different as the wavelength is short enough compared with the length of the transmission lines, i.e., the electrical potential are not equal to each other on the transmission lines. For example, a transmission line of λ/4 length is an important distributed parameter component in a microwave circuit. As shown in FIG. 3, the transmission from Port 1 to Port 2 is the primary signal path. There is a transmission line of λ/4 length shunted to ground on the primary signal path. The transmission line of λ/4 length will not affect the microwave signals because, although the terminal of the transmission line is shorted to ground, the point at a distance of λ/4 length from the terminal must be open. Unlike the situation in a low frequency circuit, the microwave signals in FIG. 3 will not be transmitted to the grounding point and disappear, but be transmitted smoothly from Port 1 to Port 2 or vice versa.
Based on such characteristic of the transmission line, when the grounding point is replaced by a switch, the whole circuit will be a microwave switch circuit. As shown in FIG. 4, an SPDT microwave switch in the prior art has three ports, namely, the combining port, the first shunt port and the second shunt port, which are connected through 3 transmission lines. The 3 transmission lines feature Z0 characteristic impedance, in a star topology. When switch device SW1 is closed (shorted to ground) and SW2 is open, the first shunt port is connected to the second shunt port; when SW1 is open and SW2 is closed (shorted to ground), the combining port is connected to the first shunt port. The switch devices SW1 and SW2 can be microwave PIN diodes. It can be seen that when a microwave signal is inputted from the combining port, the SPDT microwave switch can only offer two working statuses: 1) signal available at the first shunt port and no signal at the second shunt port; 2) signal available at the second shunt port and no signal at the first shunt port, thus the switch functions properly as an SPDT microwave switch for the signal. According to low frequency circuit theory, when both SW1 and SW2 are open, the signal from the combining port will reach both the first and the second shunt ports, yet the circuit in such status demonstrates a certain disadvantage: the circuit shows inconsistent impedance at Point A (the “tri-line junction”) in FIG. 4, which means microwave signals, from whichever port they are inputted, will be reflected back to the input port and the theoretical return loss (RL) of all the ports equals −9.5 dB, which is rather dissatisfying. The RL is generally required to be −18 dB or better for conventional microwave devices, therefore the SPDT microwave switch shows low performance in power dividing and combining, proving a low value in practical application.
There is also a tendency in the prior art which includes a topology consisting of microwave electrical switch chips or microwave relays and power dividers. Though the solution achieves the function of microwave signal switching, dividing and combining, it requires complex circuits and a large number of devices, which leads to low reliability and high cost.